Automotive hydraulic control systems, such as applied for the control of a continuously variable camshaft phaser, have been proposed in which the pressure of a control fluid, such as oil, is controlled for positioning of a hydraulic actuator. Control fluid viscosity can vary significantly with fluid temperature. Accordingly, it has been proposed to directly measure control fluid temperature using a conventional thermocouple or thermistor, and to control the actuator in response to measured temperature. More specifically, it has been proposed to determine an initial control command as a function of measured control fluid temperature, and to vary control gains in response to measured variation in control fluid temperature. The transducer for measuring control fluid temperature, such as a conventional thermocouple or thermistor can add significant cost to the hydraulic control system and may not accurately reflect the relevant fluid temperature characteristic. It would therefore be desirable to provide for control of the hydraulic actuator control in response to an accurate indication of oil temperature without use of a temperature transducer, so that the transducer may be removed from the system and control accuracy improved.
The hydraulic actuator may be applied in continuously variable camshaft phaser control for varying phasing between an internal combustion engine camshaft and an internal combustion engine crankshaft. The variation in phasing is known to provide engine emissions benefits, for example through precise control of an amount of dilution of an engine cylinder inlet air-fuel charge without use of external exhaust gas recirculation systems, in an internal dilution control process. It has been established that responsive, precision phasing between the camshaft and crankshaft must be maintained under substantially all engine operating conditions to yield the emissions benefits associated with such phasing control. Accordingly, it would be desirable to precisely and responsively control the hydraulic actuator that drives the phaser position under all operating conditions without resort to direct measurement of hydraulic control fluid temperature.